1. Field of the Invention
This invention relates to a resonant-cavity light-emitting diode which emits light in a direction perpendicular to one main surface of a substrate.
2. Description of the Related Art
A resonant-cavity light-emitting diode is a light-emitting device having a structure similar to that of a vertical-cavity surface emitting laser and is operated with laser oscillation suppressed by setting the reflectance on the light-emitting side low appropriately. As this type of light-emitting device, for example, a device disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 12, pp. 1685 to 1687, Dec. 12, 1998, is known.
The resonant-cavity light-emitting diode has features in comparison with a normal LED since it has a cavity structure that (1) the emission spectral line width is narrow, (2) the directivity of emitted light is high, and (3) the carrier lifetime of spontaneous emission is short.
Therefore, the resonant-cavity light-emitting diode is extremely suitably used as a transmission light source for an optical LAN and an optical data link. It is a light transmission device which plays an important role, particularly, in the transmission rate of approximately 100 Mbps to 1 Gbps and the response characteristics thereof largely depend on the device size.
That is, as the device size becomes smaller, the carrier density of an active layer caused when the same amount of current is injected therein becomes higher, and the carrier lifetime which controls the response characteristics of the light-emitting diode becomes shorter as the carrier density becomes higher. Therefore, generally, the response characteristics of the device become higher as the device size becomes smaller.
However, a reduction in the device size means that the light-emitting area of the device is made small and there occurs a problem that light output power is lowered accordingly as the device size is reduced.
One of the main factors in lowering the light output power due to reduction in the device size is considered as follows. That is, since the resonant-cavity light-emitting diode is a device which emits light in a direction perpendicular to one main surface of the substrate on a flat plate, the light-emitting area on the main surface of the substrate is reduced with reduction in the device size. Therefore, the ratio of the area on the substrate side surface to the plane area of an effective active region which contributes to light emission becomes higher and the rate of light leaking into the substrate side surface side becomes higher.
Thus, the conventional resonant-cavity light-emitting diode has a problem that the light output power is lowered if the device size is reduced in order to enhance the response characteristics of the device. Therefore, it is required to realize a resonant-cavity light-emitting diode which generates high light output power and is excellent in its response characteristics.